1. Field of the Invention
The invention relates to the extraction of metal ions from dilute aqueous solutions and, more particularly, to a modified liquid-liquid extraction of such metal ions.
2. Description of the Prior Art
Extraction of metal ions from dilute aqueous solutions is important in recovery of uranium, copper and many other metal values for commercial use. It is also important in purification of water, for example, from mercury ion. Extraction is often done by liquid-liquid phase distribution using a dilute solution of a complexer in a water-insoluble oil or by contacting the feedstream with a solid ion-exchange resin. A problem with solid resins is that they are easily fractured into fines which then tend to plug the extraction system. Further, the presence of solids such as mud, grit and other debris implies a filtration step, since otherwise the solids will plug the resin bed. Liquid extraction is much faster than solid resin use, but requires large volumes of oil phase, especially for dilute metal ion solutions. Also, as with solid ion exchange, the presence of any solids usually requires filtration prior to extraction.
A number of approaches have been proposed to circumvent these problems. For example, in U.S. Pat. No. 3,102,782, a process is disclosed for the recovery of heavy metal values (uranium) from aqueous solutions by solvent extraction with novel gel-like water-insoluble solvent-containing resin compositions.
German Offenlegeschrift No. 2,732,800, (English equivalent as Israeli patent application No. 050,120 [1976]) discloses impregnated with a selective extraction agent dissolved in solvent readily absorbed by the polymer.
U.S. Pat. No. 3,320,033 discloses an extractant for metal values dissolved in aqueous media comprising a solid substance having an inert hydrophobic surface on which an inert hydrophobic surface on which an organic solvent extractant is absorbed. Essentially, the patent discloses liquid-liquid extraction in which a water-insoluble organic liquid is supported as a coating on the solid substance. An advantage of this approach is that solutions containing solids may be treated without filtration; however, the extracting ability of the system is limited to the amount of organic liquid which is absorbed on the solid substance. As disclosed in the patent, the maximum amount of organic liquid that can be absorbed in one or two monolayers (about 15 to 30 Angstroms in thickness).
A problem with the approach presented by the foregoing disclosures is that the particle size of the resins is often fixed. Further, long equilibration times are encountered, probably as a consequence of low interfacial area between liquid adsorbed on the surface of the particles and the feedstream.
A need remains in the field of hydrometallurgical extraction and dilute metals extraction for a system capable of rapid extraction of metal values and capable of tolerating solids in the feedstream.